An automobile engine generally includes a timing transmission which utilizes an endless, traveling, flexible, transmission medium such as a roller chain, silent chain, toothed belt, or the like, to transmit rotation from the engine crankshaft to one or more camshafts. In such a transmission, sliding contact guides are used to guide the transmission medium. Typically, a fixed guide is disposed in sliding engagement with the tension side of a transmission chain, i.e., the side moving from a driven sprocket to the driving sprocket, and a movable guide is disposed in sliding engagement with the slack side of the chain, i.e., the side moving from the driving sprocket to a driven sprocket. The guides control the path of the chain to prevent vibration, both in the plane of the chain and in directions transverse to the plane of movement of the chain.
A sliding contact guide typically comprises a chain-contacting shoe composed of a synthetic resin mounted on a support, sometimes referred to as a “base frame.” In the case of a movable guide, the supporting base typically has a mounting hole fitting onto a shaft extending from the engine block, and is pivotable about the shaft. The movable guide cooperates with a tensioner to maintain appropriate tension in the chain. A conventional transmission of the kind described above is depicted and explained in detail in the U.S. Pat. No. 6,086,498, granted Jul. 11. 2000.
The supporting base is typically a die cast aluminum member having an I-shaped cross-section for strength, rigidity and light weight. Because the transmission medium can exert forces on the shoe in various directions, the base frame must be rigidly attached to the shoe in order to provide proper support. The base frame is manufactured by die casting aluminum (which, as used herein, includes aluminum alloys).
In aluminum casting, forming interior surfaces of a die with a slight angle or draft is usually necessary in order to make it possible to remove the casting from the die. In manufacturing a conventional base frame for a chain guide, two separable die parts are brought together to form a cavity. As shown in FIG. 7, the guide 500 comprises a cast aluminum base frame 520 and a resin shoe 510 mountable on the front surface 521 of the base frame. The base frame has a generally I-shaped cross-section, throughout most of its lengths, with short reinforcing ribs extending from the front flange to the back flange of the base frame on both sides of the web of the base frame. A boss, formed adjacent one end of the base frame, has a mounting hole 522 for receiving a shaft on which the guide can be pivoted.
As shown in FIG. 7, each of two opposed die parts P1 and P2 forms one-half of an I-shaped cavity conforming to the I-shaped cross-section of the base frame. The die parts meet at location a parting line PL, which intersects the central parts of the front and back surfaces of the base frame 520 formed by the die. Consequently, it is necessary to form two oppositely sloped drafts in the respective die parts P1 and P2, meeting at a peak 521 on the front surface of the base frame. A similar pair of oppositely sloped drafts is formed on the die parts where they form the back surface of the base frame. The oppositely sloped drafts allow the die parts to be separated easily for release of the casting. However, following the casting step, since the front surface of the base frame has a peak, it must be machined in order to ensure a close fit between the resin shoe and the base frame. Especially since the front surface of the base frame is curved in its longitudinal direction, machining of the front surface is time-consuming and difficult, and increases the cost of manufacture.
AS shown in FIG. 8, conventional die casting also produces a double taper in the mounting hole formed in boss 522. Each of the die parts P1 and P2 is formed with an annular cavity surrounding a central protrusion. The protrusions meet at a parting line PL that intersects the middle of the mounting hole in boss 522. The protrusions of the die parts are oppositely tapered as seen in FIG. 8, to provide drafts allowing the die parts P1 and P2 to be separated from each other for removal of the casting. A doubly tapered mounting hole will inevitably allow yaw and rolling motion of the guide when in use, resulting in rapid wear and reduced control of vibration in the transmission chain that slides on the guide. Consequently a costly, separate, precision machining step is required to straighten the interior wall of the mounting hole.
In addition, especially in the case where the boss portion of the guide base is comparatively long, torsion is liable to occur during the casting process the wall of the boss must be formed initially with a thickness sufficient to allow a straight hole to be formed in the subsequent machining step. The requirement for increased thickness can result in the production of voids or “blow holes” in the casting process. These voids can impair the strength and endurance of the guide, and result in failure due to the formation of cracks.
In the conventional casting process, the die parts are separated only after the aluminum casting has completely cooled. Thermal shrinkage during the cooling process can lead to the application of excessive loads to the die parts as they are separated, and shorten the useful life of the die.
The principal objects of this invention include one or more of the following: avoidance of the above-mentioned problems encountered in conventional methods of die casting; obviating, or at least reducing the cost of, machining steps carried out following die casting; provision of a guide base that pivots smoothly on a mounting shaft; provision of an easily manufactured guide base on which a shoe can be supported without wobbling; reduction of production costs and weight of the base frame; and increasing the life of the mold. Other objects and advantages of the invention will be apparent to persons skilled in the art in the light of the following description, when read in conjunction with the drawings.